1. Field of the Invention
This invention relates to a method for preparing an aromatic isocyanate.
2. Description of the Prior Art
Several methods have hitherto been developed for preparing aromatic isocyanates of which only one, however, has achieved commercial importance. Essentially all of such isocyanates of commerce are currently produced via the phosgenation of an aromatic amine. Since the appropriate amine must be first derived from the naturally occurring aromatic hydrocarbon, the overall reaction scheme is relatively complex from a processing standpoint. Also the manufacturing facility for implementing the method is capital intensive. In light of the high fixed costs involved in the current manufacture of these commodities, which are finding ever increasing industrial usages, the development of a more economical process is patently indicated.
The genesis of the approach to prepare isocyanates in accordance with the chemistry concerned herein is to be found in a number of related papers which began to appear in the early part of the last decade. One of the forerunners of these articles is Unland, M. L., J. Phys, Chem, 77, 1952 (1973). The latter reported work was directed to a study of the reductive removal of nitrogen oxides from automobile exhaust through the agency of a noble metal catalytic converter system. The most significant finding of this study was that carbon monoxide and nitric oxide combine at elevated temperatures to form a complexed isocyanate species on the surface of a noble metal catalyst.
The first potentially industrial application of the above finding is set forth in U.S. Pat. No. 4,174,377. Therein it taught that a variety of cyanate compounds including isocyanic acid can be prepared directly by contacting a noble metal hydrogenation catalyst at an elevated temperature with a feed stream of nitrogen oxide, carbon monoxide and a source of hydrogen. Although to date isocyanic acid has only limited industrial applications, the foregoing advance in the art nonetheless serves to conjure up the desideratum of preparing an aromatic isocyanate in such a chemically elegant manner.